Optical fiber technology has been rapidly developing over the last few years. Fibers having loss below 1 dB per kilometer at a predetermined operating wavelength, and capable of transmission rates in excess of 10 gigabits kilometer per second at that wavelength are now routinely being produced. To achieve such operating characteristics, fabrication methods have been highly refined to minimize contamination, and fiber designs have been introduced that minimize such properties as scattering loss and dispersion.
Optical fibers typically comprise a central region, the core, having a refractive index that is greater than the refractive index of the material surrounding the core, usually referred to as the cladding. Both core and cladding generally comprise silica as a major constituent, typically comprising more than 80% b.w. SiO.sub.2. The refractive index of silica is changed as required by means of doping with appropriate chemical elements. For instance, doping of silica with germania raises the refractive index, whereas doping with fluorine or boron results in index lowering. Optical fiber is generally produced by drawing from a so-called preform, a glass body produced by a process comprising deposition of in situ-formed glassy material onto a substrate.
In one category of preform manufacturing techniques, the so-called internal deposition techniques, the substrate is a preexisting silica tube, with the glassy material deposited onto the inside surface of the tube. See J. B. MacChesney, Proceedings of the IEEE, Vol. 68(10), 1980, pp. 1181-1183. In another category, to be referred to as "outside" deposition techniques, the deposition substrate typically is the outside of a mandrel or the endface of a starting silica rod. See T. Izawa and N. Inagaki, op. cit., pp. 1184-1187, and P. C. Schultz, op. cit., pp. 1187-1190.
For inside deposition processes, e.g., the Modified Chemical Vapor Deposition (MCVD) process, or the Plasma Chemical Vapor Deposition (PCVD) process, gaseous precursors (e.g., SiCl.sub.4, GeCl.sub.4), oxidants (typically O.sub.2), and possibly diluents are introduced into the bore of a substrate tube, and a portion of the internal volume is heated, such as by means of an external heat source or a plasma. In the hot zone a chemical reaction takes place, resulting in formation of an amorphous deposit, which is consolidated into glass by heating of the substrate.
In outside deposition processes, e.g., the Vapor Axial Deposition (VAD) process or the Outside Vapor Deposition (OVD) process, the gaseous precursors are reacted in a flame, and the resulting amorphous reaction product deposited on the substrate. After completion of the deposition the deposited material is "dried" and sintered to a glass body. Frequently, a silica sleeve tube is shrunk around the glass body formed by deposition, to improve process economies.
Typically, in inside deposition processes, the gas flow into the reaction zone is regulated such that the material deposited onto the inside wall of the substrate tube has a relatively low refractive index (including an index matched to the substrate index), and last-deposited material has a relatively high refractive index. The former material will collectively be referred to as deposited cladding material, the latter as core material. The refractive index of the deposited cladding need not be uniform, and fibers having a multiplicity of deposited cladding regions, including undoped as well as downdoped regions, are known. See, for instance, U.S. Pat. No. 4,439,007. Generally, the amount of deposited cladding material is a significant fraction of the total amount of deposited material, and in single mode, (SM) fiber, the amount of deposited cladding material generally greatly exceeds the amount of core material.
Fibers produced by an outside deposition process also comprise a deposited core and deposited cladding. In many cases they also comprise an outer cladding that is derived from the preexisting sleeve tube.
In SM optical fibers, a significant fraction of the total guided energy is not confined to the core, and the tail of the power distribution extends a considerable distance into the cladding material. Since the substrate or sleeve tube material typically is less pure, and therefore much more lossy, than the deposited cladding material, it is necessary to ensure that no significant fraction of the total power propagates in the substrate- or sleeve-tube-derived material. (Substrate-tube-derived and sleeve-tube-derived material will be referred to collectively as "tube-derived" material.) This is typically achieved by making the layer of deposited cladding material sufficiently thick. For instance, if the deposited cladding is of such a thickness that only 10.sup.-4 of the total power is propagating in the tube-derived material then a substrate (or sleeve) tube having a loss of 100 dB/km would only add 0.01 dB/km absorption loss to the fiber.
In one type of prior art SM fiber, the so-called depressed index cladding fiber, the effective refractive index of the core often does not substantially exceed (typically by less than about 0.5%) that of pure silica, with the effective refractive index of the deposited cladding material being substantially lower than that of the core, and also lower than that of silica. Depressed cladding SM fibers possess several advantageous features. Due to the possibility of using an undoped (or lightly doped) core, such fibers can have low Rayleigh scattering loss. Furthermore, such fibers can be radiation damage tolerant, and can be relatively immune to loss increases due to hydrogen indiffusion.
In prior art fiber with depressed index cladding a further reason exists for having a relatively thick layer of deposited cladding material. If in such fiber a nonnegligible fraction of the total power were to leak to the tube-derived cladding material, the fiber would have relatively high loss, even if the tube-derived material had a low absorption loss, comparable to that of the deposited cladding material. This type of loss is referred to as "leaky mode" loss, since the radiation propagating in the outer cladding is unguided and thus will "leak" away. Similar to absorption loss in the tube-derived cladding, leaky mode loss can be avoided by interposition of a sufficiently thick layer of deposited cladding material between core and tube-derived cladding.
Since depressed index cladding optical fiber offers advantages over other fiber designs, such fiber is of considerable interest. See, for instance, U.S. Pat. No. 4,439,007. The amount of deposited cladding material generally is a significant fraction of the total amount of deposited material in all silica-based optical fiber, but in SM fiber the amount of deposited cladding material typically greatly exceeds the amount of core material. Since deposition time during preform manufacture is a significant cost item in optical fiber production, fiber that requires less deposited cladding material than is necessary for prior art designs is therefore of considerable interest. This application discloses a fiber having tChis and other advantageous features.